Help Troubleshooting Subaru 3.6L

[Ultramaroon]

Originally Posted by Spuds

I don't see how it invalidates the theory that a clogged fuel filter is preventing flow at lower pump settings.

Possible culprits:
1. Fuel filter is clogged
2. Fuel pump is dieing
3. Some fuel pressure regulation device is malfunctioning.
4. Coils/plugs
5. Alternator (possibly also the reason the battery was shot?)

I can probably check the the Alternator voltage output pretty easily, (obd readout or multimeter) to make sure it's at 14V+ right?

That just leaves 1-4. I think it's going to be more cost/time effective to just replace the filter and pump together than try to rig something together to figure out how to measure fuel pressure in a running engine and fi more diagnosis. If that costs me $300 and an afternoon to learn that wasn't it, well I learn better the hard way anyway lol. Plus, the filter at least is overdue for replacement anyway.

If that doesn't solve the problem, the plugs are due for replacement, and then coil packs. If I am still in the dark after that, well, it might be time to pay somebody to fix it...

Edit, first, I'm going to inspect the fuel lines under the car. I wonder if they may have been damaged when I got the cat inspected...

Fuel delivery issues are still in play but I can't draw a straight line to the counter-intuitive poor performance at idle while improving under power. That we are dealing with a returnless system invalidates only my specific case in support of Tcoat's observation.

Nothing wrong with replacing the pump assembly. It would eliminate 1-3 in one fell swoop. You still have to drain the tank to replace the pump. 300 bucks would buy lots of pressure gauges.

Definitely check the charging system. Do the headlights dim at idle? Does the engine stumble when everything's turned on? Cabin fan, headlights, brake lights all at once?

It occurs to me that we haven't discussed relocating coil packs. If the misfire is consistently on 5 and 6, swap a couple. They don't last forever. Worn (or conventional) plugs are hell on coil packs.

[Spuds]

Originally Posted by Ultramaroon

Fuel delivery issues are still in play but I can't draw a straight line to the counter-intuitive poor performance at idle while improving under power. That we are dealing with a returnless system invalidates only my specific case in support of Tcoat's observation.

Nothing wrong with replacing the pump assembly. It would eliminate 1-3 in one fell swoop. You still have to drain the tank to replace the pump. 300 bucks would buy lots of pressure gauges.

Definitely check the charging system. Do the headlights dim at idle? Does the engine stumble when everything's turned on? Cabin fan, headlights, brake lights all at once?

It occurs to me that we haven't discussed relocating coil packs. If the misfire is consistently on 5 and 6, swap a couple. They don't last forever. Worn (or conventional) plugs are hell on coil packs.

Thanks for the ideas.

Fuel pump pressure is controllable electronically, so the best guess is it lowers the target pressure at lower rpm to put less strain on the pump, then steps it up as more fuel is needed. At some point that increase results in enough pressure to keep all 6 injectors going. It may not have a feedback loop in the control of the pump, so it is not adapting to increased pressure demand.

I think I might have a fuel pressure test kit from my grandfather, probably need a bunch of modern connectors though lol. What should the pressure be? I cannot seem find specs using my apparently weak google-fu. I suppose if it's lower at idle than it is primed but no flow that is evidence that the filter is the problem?

Everything electronic I can control works fine all together, and ecu voltage is still reading 14.2ish so I think charging is fine? Engine stumbles at low rpm as described, and is remarkably consistent.

Yeah I hear you on coil packs, but the chances of two different packs on different banks going at the exact same time are pretty low. I cleared the codes and checked again, still 5 and 6. I suppose it's cheaper to try to swap coils around before getting that in-tank assembly though...

[thomasmryan]

reset the codes, swap the coil packs to 1 and 3, drive and then check codes.



if it stays on 5 and 6, check the plugs. if the plug holes are swimming in oil, get the valve cover gasket kit for both sides.

[Ultramaroon]

Originally Posted by thomasmryan

if the plug holes are swimming in oil, get the valve cover gasket kit for both sides.

Is that a thing for these engines? I can picture it.

[Spuds]

Spent a few hours this morning shuffling plugs and coil packs around, using every single socket, wrench, and adaptor in my arsenal of course (ok just the 10mm and 12mm sockets and wrenches). Reset codes, still reports misfiring on 5 and 6. No visible damage to coils, plugs were dry and looked fine.

Sooo, fuel supply next?

At least I can say I've done spark plug stuff on a Subaru lol.

[Atmo]

Another long shot: The misfires are isolated to #5-6, it isn't coil packs so yeah, something's off with fuel delivery.

I recall that D4-S mapping is compound DI/Port at lower rpm, exclusively DI at midrange, then both again at the high end. So, could the #5-6 port injectors be clogged? Maybe bench testing them would help.

[thomasmryan]

see if the intake manifold bolts are tight on the drivers side. (it's an o-ring type gasket.)

[Spuds]

Originally Posted by Atmo

Another long shot: The misfires are isolated to #5-6, it isn't coil packs so yeah, something's off with fuel delivery.

I recall that D4-S mapping is compound DI/Port at lower rpm, exclusively DI at midrange, then both again at the high end. So, could the #5-6 port injectors be clogged? Maybe bench testing them would help.

Why would a Subaru car without Toyota's D4S have this problem?

[Spuds]

Originally Posted by thomasmryan

see if the intake manifold bolts are tight on the drivers side. (it's an o-ring type gasket.)

Cylinders 5 and 6 are on opposite banks...

[Atmo]

Originally Posted by Spuds

Why would a Subaru car without Toyota's D4S have this problem?

That's putting it nicely!

[thomasmryan]

Originally Posted by Spuds

Cylinders 5 and 6 are on opposite banks...

yes, i finally found the diagram in the manual. a google fail on my part.





the 06 manual shows a drain plug on the tank.

[mrg666]

coil packs
spark plugs
injectors
valve springs

My FR-S was checked in that order for the exact same problem. It was valve springs.

[Spuds]

Originally Posted by mrg666

coil packs
spark plugs
injectors
valve springs

My FR-S was checked in that order for the exact same problem. It was valve springs.

Well, I just ordered a fuel pump assembly, but good to know if that doesn't work.

[mrg666]

Originally Posted by Spuds

Well, I just ordered a fuel pump assembly, but good to know if that doesn't work.

I very much hope it is the fuel pump. Replacing valve springs requires engine taken out. Good luck.